1. Field of the Invention
The present invention is a system and process for preventing robotic systems from transferring two blank work pieces, i.e., a double blank, from a stack of blanks to the destination for the blank.
2. General Background and State of the Art
The manufacturing process of stamping, forming and assembling sheet metal components often needs to pick a piece of sheet metal automatically from a stack and load it into a machine for processing. The sheet metal may be blanks picked up from a stack of blanks for loading into the first machine in a line of subsequent stamping, forming or other operations. Alternatively, the sheet metal can be already stamped to form a stack of identical parts, which are loaded into another machine. Though some processes manually pick the top sheet from a stack, the present invention deals with an automated process.
Automated processes for picking a sheet from a stack use either a six-axis programmable robotic manipulator or so-called xe2x80x9chard automationxe2x80x9d or xe2x80x9cfixed automation.xe2x80x9d The latter means a two- or three-axis device for picking from a first point and placing at a second point. Either method of manipulating parts requires an end-of-arm-tool, or EOAT, which grips, holds and releases the piece so the manipulator can pick the top sheet from the stack and move it to its destination. The technology of choice for the EOAT of most applications is vacuum suction cup gripping. Using compressed air and a vacuum pump or venturi type vacuum generating device, rubber suction cups come in contact with and adhere to the part due to the vacuum. When the manipulator has picked and moved the part to its destination, the pressure is reversed and the part is xe2x80x9cblown-off,xe2x80x9d or released from the suction cups. Hardwick and Littlewood, xe2x80x9cSheet Material feeding and Unloading Apparatus,xe2x80x9d U.S. Pat. No. 5,848,785 (1998), describes such a process. This technology is well proven and in use in thousands of applications.
Occasionally, the manipulator picks a blank from the stack and the next blank in the stack sticks to the first sheet. This is referred to as a double blank condition. Factors that can cause the blanks to stick together include oil or other substances, corrosion, dirt and static electricity. Mechanical interlocking of small metal burrs especially on processed parts also cause double blanks. If the stamping, forming, welding or other process calls for a single blank, loading a double blank would be undesirable. It wastes money, generates scrap parts, causes lost production time, and damages equipment. The challenge of preventing a double blank from being loaded into a subsequent process is a major problem for the metal forming industry or other industries that handle flat sheets of material.
The metal forming industry employs methods to address this problem:
1. Fanner magnets: These are powerful magnets that are brought within close proximity of the edge of the parts in a stack. The magnets cause the rest of the stack to repel the top blank in the stack. They only work with steel or other magnetic material, not aluminum, plastic, or other non-magnetic material.
2. Air knives: These are nozzles that direct a high pressure stream of air at the edge of the blanks to displace the parts with air pressure enough to break the bond with the next part. Air knives are not always successful.
3. Double blank detectors: They are electronic sensing devices that can mount on the EOAT. The face of the sensor is forced flush with the surface and electronically detects if two blanks are present. When the system detects a double blank, the system ejects both blanks to a separate location.
Fanner magnets and air knives act on the stack itself and are passive. They can give the process the best chance of avoiding double blanks, but they are not completely reliable. Once the robot picks a double blank and moves it away from the stack, the fanner magnets or air knives do not work and are not in a position to work. Without manual intervention, a double blank may be loaded.
Detectors have different problems. They only detect. If the double blank detector detects a double blank, it can send a signal to a control device to initiate an action. Nevertheless, the detector itself cannot correct the double blank condition. When the process control device of some applications receives a signal that a double blank condition exists, it signals the manipulator to shake the EOAT trying to unstick the stuck blank from the gripped blank. This approach is very primitive and unreliable. When it works, the dislodged blank often lands in an incorrect location or orientation. Therefore, a person must intervene to return the dislodged blank to the stack of blanks or to the workstation.
Prior patents disclose some of these concepts. Moltrasio et al., xe2x80x9cSystem for Picking Up and Separating Bags from a Stack to be Applied to Automatic Apparatus,xe2x80x9d U.S. Pat. No. 4,516,762 (1985), discloses an upper unit with downwardly facing suction cups to lift a paper bag from a stack of bags. xe2x80x9cUnsticking unitsxe2x80x9d consisting of bars separate any bags beneath the one being picked up. Sartorio et al., xe2x80x9cMethod of Unstacking Metal Sheets,xe2x80x9d U.S. Pat. No. 4,806,071 (1989), has two side-by-side units. The units can move laterally relative to an unloading plate. The plate slides back and forth. Each upper element can pick up a metal sheet from a stack using suction cups. If the device detects only one blank, the upper element lowers the blank onto a lower element. If the device detects three or more blanks, it has a method for rejecting a larger stack. If it detects only two plates, i.e., a double blank, the device separates the two plates.
Previously-mentioned U.S. Pat. No. 5,848,785 uses downwardly facing suction cups for lifting a blank from a stack. A sensor checks whether only a single blank has been lifted or whether a second sheet also has been lifted. If the sensor detects a second sheet, upwardly facing vacuum cups on a frame are moved into an operative position below the second sheet. Those upwardly facing cups pull the double blank away from the first blank.
Solving problems of prior art devices is a principal object of the present invention. Another object of the present invention is to disclose a system and method for separating double blanks from each other. Another object of the present invention is to accomplish separation automatically. Another object of the present invention is to have a double blank separation station as a separate, stationary unit to be accessed by a preexisting programmed robotic manipulator. These and other objects will be apparent to those skilled in the art.
The present invention uses a manipulator to transfer a blank from a stack of blanks to a workstation. The manipulator transfers a blank first to a double blank separation station for separation. If the manipulator picks a double blank, the two blanks are separated at the separation station. The upper blank stays on the manipulator, but the lower blank stays at the station. Then, the manipulator moves the upper blank to the workstation. The manipulator then returns to the station to pick up the lower blank, which had stayed at the station. Then the manipulator carries the lower blank to the workstation. Finally, the manipulator returns to the stack of blanks for additional blanks.
The double blank separation station of the present invention is intended to be used with a programmable robotic manipulator having an EOAT that has means for gripping and holding a blank to be removed from a stack and moved to its destination. The gripping means usually is vacuum suction cups for picking up the blank or part to be moved. Reversing the vacuum xe2x80x9cblows offxe2x80x9d or releases the blank from the suction cups.
If the suction cups on the EOAT pick up a double blank, i.e., two blanks stuck together, the double blank separation station of the present disclosure can be accessed for separating the blanks. The station in the exemplary embodiment comprises inverted suction cup, which mate with the underside of the part being picked. The double blank separation station can be incorporated into the process in either an xe2x80x9cevery cyclexe2x80x9d or xe2x80x9cdouble blank detectedxe2x80x9d manner. In the every cycle approach, the station is accessed during every cycle. In the xe2x80x9cdetectedxe2x80x9d approach, the station is accessed only when a double blank situation is detected.
In either case, the robotic manipulator moves the blank to the double blank separation station onto the upward facing suction cups. With the upper and lower suction cups in the xe2x80x9cvacuum onxe2x80x9d mode, the manipulator moves upwardly from the double blank separation station by {fraction (1/16)} or xe2x85x9 inch (1.5-3 mm). If there are two blanks, the force separates them. If there is only one blank, the natural compliance of the rubber cups will accommodate this much movement without letting go top or bottom. Thereafter, the lower cup vacuum of the double blank separation station is turned off and the robot is free to move a single blank to the destination. If there were actually two blanks, gravity retains the separated blank at the station.
The double blank separation station is equipped with a part-sensing device such as a proximity switch or mechanical limit switch that notifies the controller of the robot that a blank now resides on the station. On a next cycle, the manipulator will return to the double blank station instead of the stack of blanks to pick up the separated blank before it resumes its normal process of accessing the stack of blanks.
In a double blank detected mode, the robotic manipulator picks up the blank(s) from the stack. When the blank moves clear, a cell controller looks for a signal from a double blank detector that a double blank is present. If such is the case, the robotic manipulator moves the blank(s) to the double blank separation station. If the detector senses a single blank, the manipulator transports the block directly to the workstation, bypassing the separator
One of the advantages of the present invention is that the double blank separation station is a separate unit that can be incorporated when needed with preexisting conventional multi-axis programmable robotic manipulators where provisions for blank separation were not incorporated in the original design of the components.